1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device, and more particularly to a nitride semiconductor light emitting device having patterns formed on the upper and lower surfaces of a substrate wherein the patterns are capable of changing light inclination at the upper and lower surfaces of the substrate to decrease total reflection, thereby improving light emitting efficiency. Also, the present invention relates to a method of manufacturing the same.
2. Description of the Related Art
Lately, a nitride semiconductor, such as GaN, has been widely used as a photoelectric material or an essential material for electronic devices by virtue of its excellent physical and chemical properties.
In case of a GaN-based light emitting device, a red light emitting diode has been commercially used since the second half of the 1960's. GaN-based blue and green light emitting diodes have been commercially used since the second half of the 1990's. Also, a white light emitting diode can be formed from a GaN-based compound semiconductor. As the high-efficiency three primary colors (red, blue, and green) and white light emitting diodes have appeared, the application range of the light emitting diode has expanded. For example, the light emitting diodes are widely used in various applications, such as a backlight for keypads and liquid crystal display units, a traffic light, a guiding light for airport runaways, a head light for airplanes or automobiles, and an illuminating light.
Especially, the GaN-based white emitting diode now has a light emitting efficiency of approximately 25 lm/W, which is still less than that of a fluorescent lamp. However, the performance of the GaN-based white emitting diode is being rapidly improved. Consequently, it is expected that the GaN-based white emitting diode will substitute for an incandescent lamp as well as the fluorescent lamp sooner or later.
FIG. 1 shows the basic structure of such a GaN-based semiconductor light emitting device. Referring to FIG. 1, the GaN-based semiconductor light emitting device comprises an n-type nitride semiconductor layer 12 formed on a sapphire substrate 11, an active layer 13, having a multi well structure, formed on the n-type nitride semiconductor layer 12, and a p-type nitride semiconductor layer 14 formed on the active layer 13. The p-type nitride semiconductor layer 14 and the active layer 13 are partially etched such that the upper surface of the n-type nitride semiconductor layer 12 is partially exposed. On the upper surface of the p-type nitride semiconductor layer 14 is formed a p-electrode 15 (hereinafter, referred to as “P-electrode”), and on the exposed upper surface of the n-type nitride semiconductor layer 12 is formed an n-side electrode 16 (hereinafter, referred to as “N-electrode”).
The structure shown in FIG. 1 is the basic structure of the GaN-based semiconductor light emitting device. In addition, a buffer layer may be disposed between the n-type nitride semiconductor layer 12 and the substrate 11 for improving lattice latching. Also, a transparent electrode layer (T metal) 16 may be disposed between the p-type nitride semiconductor layer 14 and the P-electrode 15 for forming ohmic contact and improving current injection efficiency.
The above-mentioned light emitting devices, especially the GaN-based white light emitting device, have low light emitting efficiency, which is the greatest problem with the light emitting devices. Generally, light emitting efficiency is determined by efficiency at which light is generated, efficiency at which light is emitted from the light emitting device, and efficiency at which light is amplified by means of a fluorescent substance. The conventional GaN-based white light emitting device has a problem in that the efficiency at which light is emitted from the light emitting device is very low. The major obstacle to emission of light from the light emitting device is internal total reflection. Due to the difference of refractive indices between the respective layers of the light emitting device, the amount of light exiting from the interfaces corresponds to approximately 20% of the total amount of light. Furthermore, the remaining light having not exited from the interfaces moves in the light emitting device, and finally decays into heat. As a result, the amount of heat generated from the light emitting device increases while light emitting efficiency of the light emitting device is low, and therefore the service life of the light emitting device is reduced.
Alternatively, the light emitting device shown in FIG. 1 may be turned upside down such that the light emitting device has a flip-chip bonding structure. Also, the light emitting device may be provided with a rear surface reflecting film such that light generated toward the electrodes 15 and 16 is reflected to the substrate 11. In this case, the light is emitted through the substrate 11. Consequently, it is possible to make the best use of light reduced due to the low transmission efficiency of the P-electrode 15, by which the light emitting efficiency is increased by approximately 40%. However, the problem that the light emitting efficiency is reduced due to the total reflection in the light emitting device has yet to be solved.